Both natural and synthetic elastomeric latex compositions and blends thereof have been employed in the preparation of foam products, such as foam backings for tufted carpets. Typically, the preparation of a latex foam has required a multicomponent latex composition wherein a latex user has been required to mix the component parts of the latex composition just prior to use. The latex composition so mixed is then foamed by mechanically whipping in air with, for example, an Oakes foamer, and then depositing the latex froth on a surface, such as a back of a carpet, a belt or a mold, and heating to effect the desired gel embossing and cure of the foam. Some disadvantages of such conventional latex foam manufacture have been the necessity of shipping a multiple-part mixture to the customer and requiring the customer to mix the component parts just prior to use, as well as to employ a foamer device containing proportioning pumps.
The division of a foamable latex into its component parts for shipment to a customer may involve two, three, four or even more different component compositions. A supplier might furnish a latex customer a multicomponent foam packaging comprising four separate components. A first component would comprise a natural or synthetic gellable, vulcanizable elastomeric latex, such as natural rubber or a styrene-butadiene rubber, with a foaming agent, such as surfactants or frequently potassium soaps. A second component would comprise various vulcanizing or curing agents (and cross-linking ingredients where required), together with an accelerator or combinations of accelerators known as a curing system. Some accelerators would include metal alkyl xanthates, salts of N-substituted dithio carbamic acid, thiazoles, thiurams, and other compounds. Commonly used as accelerators are the zinc salts of dialkyl dithio carbamates. Optionally, antioxidants and stabilizers may be added, and where necessary, a cross-linking agent used, for example, in carboxylated latices, a melamine or urea-formaldehyde resin or isocyanate compound, or peroxides used. The materials of the second component, if included in the first component, would result in a precure or premature gellation of the mixed components so that when the foam is prepared, it is friable when deposited or would not be capable of being embossed. In addition, the mixture of the first and second components would not permit long-term storage without and undesirable gellation or precure, or both. Gellation refers to the change of a latex system from a liquid to a solid, while precure refers to the premature change in the elastomeric and chemical structure and properties of the polymer. Both gellation and precure may effect the storage life and ultimate use of a latex composition.
The third component is typically a gelling or coagulating agent which lowers the pH of the latex composition and destabilizes the latex to obtain coagulation after foaming through a chemical reaction. Gelling agents employed would include a fluoride, such as sodium silicofluoride, or a hydrolizable salt such as an ammonium salt such as ammonium acetate. An optional fourth component would include fillers and pigments which may be supplied by the manufacturer in dispersion form for ultimate mixing by the customer or in dry form for dispersion and mixing by the customer. Such multicomponent systems have many disadvantages associated with their preparation, shipment, and ultimate use. It is desirable, therefore, to provide a latex composition and method which would overcome such disadvantages, and particularly, to provide a foamable latex composition which could be shipped and stored for extended periods of time without precure or gellation.
U.S. Pat. No. 3,491,033 describes a two-part latex composition for foaming wherein the latex is specially compounded so as to be free of agents capable of gelling the wet foam. Such no-gell foam compositions have the disadvantage of generally requiring a single pass oven for their use as they depend on water removal to gell.